Send-to-device support (#1072)

* Groundwork for send-to-device messaging

* Update sample config

* Add unstable routing for now

* Send to device consumer in sync API

* Start the send-to-device consumer

* fix indentation in dendrite-config.yaml

* Create send-to-device database tables, other tweaks

* Add some logic for send-to-device messages, add them into sync stream

* Handle incoming send-to-device messages, count them with EDU stream pos

* Undo changes to test

* pq.Array

* Fix sync

* Logging

* Fix a couple of transaction things, fix client API

* Add send-to-device test, hopefully fix bugs

* Comments

* Refactor a bit

* Fix schema

* Fix queries

* Debug logging

* Fix storing and retrieving of send-to-device messages

* Try to avoid database locks

* Update sync position

* Use latest sync position

* Jiggle about sync a bit

* Fix tests

* Break out the retrieval from the update/delete behaviour

* Comments

* nolint on getResponseWithPDUsForCompleteSync

* Try to line up sync tokens again

* Implement wildcard

* Add all send-to-device tests to whitelist, what could possibly go wrong?

* Only care about wildcard when targeted locally

* Deduplicate transactions

* Handle tokens properly, return immediately if waiting send-to-device messages

* Fix sync

* Update sytest-whitelist

* Fix copyright notice (need to do more of this)

* Comments, copyrights

* Return errors from Do, fix dendritejs

* Review comments

* Comments

* Constructor for TransactionWriter

* defletions

* Update gomatrixserverlib, sytest-blacklist

syncapi/consumers/eduserver_sendtodevice.go

@@ -0,0 +1,113 @@
+// Copyright 2020 The Matrix.org Foundation C.I.C.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package consumers
+
+import (
+	"context"
+	"encoding/json"
+
+	"github.com/Shopify/sarama"
+	"github.com/matrix-org/dendrite/eduserver/api"
+	"github.com/matrix-org/dendrite/internal"
+	"github.com/matrix-org/dendrite/internal/config"
+	"github.com/matrix-org/dendrite/syncapi/storage"
+	"github.com/matrix-org/dendrite/syncapi/sync"
+	"github.com/matrix-org/dendrite/syncapi/types"
+	"github.com/matrix-org/gomatrixserverlib"
+	"github.com/matrix-org/util"
+	log "github.com/sirupsen/logrus"
+)
+
+// OutputSendToDeviceEventConsumer consumes events that originated in the EDU server.
+type OutputSendToDeviceEventConsumer struct {
+	sendToDeviceConsumer *internal.ContinualConsumer
+	db                   storage.Database
+	serverName           gomatrixserverlib.ServerName // our server name
+	notifier             *sync.Notifier
+}
+
+// NewOutputSendToDeviceEventConsumer creates a new OutputSendToDeviceEventConsumer.
+// Call Start() to begin consuming from the EDU server.
+func NewOutputSendToDeviceEventConsumer(
+	cfg *config.Dendrite,
+	kafkaConsumer sarama.Consumer,
+	n *sync.Notifier,
+	store storage.Database,
+) *OutputSendToDeviceEventConsumer {
+
+	consumer := internal.ContinualConsumer{
+		Topic:          string(cfg.Kafka.Topics.OutputSendToDeviceEvent),
+		Consumer:       kafkaConsumer,
+		PartitionStore: store,
+	}
+
+	s := &OutputSendToDeviceEventConsumer{
+		sendToDeviceConsumer: &consumer,
+		db:                   store,
+		serverName:           cfg.Matrix.ServerName,
+		notifier:             n,
+	}
+
+	consumer.ProcessMessage = s.onMessage
+
+	return s
+}
+
+// Start consuming from EDU api
+func (s *OutputSendToDeviceEventConsumer) Start() error {
+	return s.sendToDeviceConsumer.Start()
+}
+
+func (s *OutputSendToDeviceEventConsumer) onMessage(msg *sarama.ConsumerMessage) error {
+	var output api.OutputSendToDeviceEvent
+	if err := json.Unmarshal(msg.Value, &output); err != nil {
+		// If the message was invalid, log it and move on to the next message in the stream
+		log.WithError(err).Errorf("EDU server output log: message parse failure")
+		return err
+	}
+
+	_, domain, err := gomatrixserverlib.SplitID('@', output.UserID)
+	if err != nil {
+		return err
+	}
+	if domain != s.serverName {
+		return nil
+	}
+
+	util.GetLogger(context.TODO()).WithFields(log.Fields{
+		"sender":     output.Sender,
+		"user_id":    output.UserID,
+		"device_id":  output.DeviceID,
+		"event_type": output.Type,
+	}).Info("sync API received send-to-device event from EDU server")
+
+	streamPos := s.db.AddSendToDevice()
+
+	_, err = s.db.StoreNewSendForDeviceMessage(
+		context.TODO(), streamPos, output.UserID, output.DeviceID, output.SendToDeviceEvent,
+	)
+	if err != nil {
+		log.WithError(err).Errorf("failed to store send-to-device message")
+		return err
+	}
+
+	s.notifier.OnNewSendToDevice(
+		output.UserID,
+		[]string{output.DeviceID},
+		types.NewStreamToken(0, streamPos),
+	)
+
+	return nil
+}

syncapi/storage/interface.go

@@ -55,10 +55,12 @@ type Database interface {
 	// sync response for the given user. Events returned will include any client
 	// transaction IDs associated with the given device. These transaction IDs come
 	// from when the device sent the event via an API that included a transaction
-	// ID.
-	IncrementalSync(ctx context.Context, device authtypes.Device, fromPos, toPos types.StreamingToken, numRecentEventsPerRoom int, wantFullState bool) (*types.Response, error)
-	// CompleteSync returns a complete /sync API response for the given user.
-	CompleteSync(ctx context.Context, device authtypes.Device, numRecentEventsPerRoom int) (*types.Response, error)
+	// ID. A response object must be provided for IncrementaSync to populate - it
+	// will not create one.
+	IncrementalSync(ctx context.Context, res *types.Response, device authtypes.Device, fromPos, toPos types.StreamingToken, numRecentEventsPerRoom int, wantFullState bool) (*types.Response, error)
+	// CompleteSync returns a complete /sync API response for the given user. A response object
+	// must be provided for CompleteSync to populate - it will not create one.
+	CompleteSync(ctx context.Context, res *types.Response, device authtypes.Device, numRecentEventsPerRoom int) (*types.Response, error)
 	// GetAccountDataInRange returns all account data for a given user inserted or
 	// updated between two given positions
 	// Returns a map following the format data[roomID] = []dataTypes
@@ -104,4 +106,26 @@ type Database interface {
 	StreamEventsToEvents(device *authtypes.Device, in []types.StreamEvent) []gomatrixserverlib.HeaderedEvent
 	// SyncStreamPosition returns the latest position in the sync stream. Returns 0 if there are no events yet.
 	SyncStreamPosition(ctx context.Context) (types.StreamPosition, error)
+	// AddSendToDevice increases the EDU position in the cache and returns the stream position.
+	AddSendToDevice() types.StreamPosition
+	// SendToDeviceUpdatesForSync returns a list of send-to-device updates. It returns three lists:
+	// - "events": a list of send-to-device events that should be included in the sync
+	// - "changes": a list of send-to-device events that should be updated in the database by
+	//      CleanSendToDeviceUpdates
+	// - "deletions": a list of send-to-device events which have been confirmed as sent and
+	//      can be deleted altogether by CleanSendToDeviceUpdates
+	// The token supplied should be the current requested sync token, e.g. from the "since"
+	// parameter.
+	SendToDeviceUpdatesForSync(ctx context.Context, userID, deviceID string, token types.StreamingToken) (events []types.SendToDeviceEvent, changes []types.SendToDeviceNID, deletions []types.SendToDeviceNID, err error)
+	// StoreNewSendForDeviceMessage stores a new send-to-device event for a user's device.
+	StoreNewSendForDeviceMessage(ctx context.Context, streamPos types.StreamPosition, userID, deviceID string, event gomatrixserverlib.SendToDeviceEvent) (types.StreamPosition, error)
+	// CleanSendToDeviceUpdates will update or remove any send-to-device updates based on the
+	// result to a previous call to SendDeviceUpdatesForSync. This is separate as it allows
+	// SendToDeviceUpdatesForSync to be called multiple times if needed (e.g. before and after
+	// starting to wait for an incremental sync with timeout).
+	// The token supplied should be the current requested sync token, e.g. from the "since"
+	// parameter.
+	CleanSendToDeviceUpdates(ctx context.Context, toUpdate, toDelete []types.SendToDeviceNID, token types.StreamingToken) (err error)
+	// SendToDeviceUpdatesWaiting returns true if there are send-to-device updates waiting to be sent.
+	SendToDeviceUpdatesWaiting(ctx context.Context, userID, deviceID string) (bool, error)
 }

syncapi/storage/postgres/send_to_device_table.go

@@ -0,0 +1,171 @@
+// Copyright 2019-2020 The Matrix.org Foundation C.I.C.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package postgres
+
+import (
+	"context"
+	"database/sql"
+	"encoding/json"
+
+	"github.com/lib/pq"
+	"github.com/matrix-org/dendrite/internal"
+	"github.com/matrix-org/dendrite/syncapi/storage/tables"
+	"github.com/matrix-org/dendrite/syncapi/types"
+)
+
+const sendToDeviceSchema = `
+CREATE SEQUENCE IF NOT EXISTS syncapi_send_to_device_id;
+
+-- Stores send-to-device messages.
+CREATE TABLE IF NOT EXISTS syncapi_send_to_device (
+	-- The ID that uniquely identifies this message.
+	id BIGINT PRIMARY KEY DEFAULT nextval('syncapi_send_to_device_id'),
+	-- The user ID to send the message to.
+	user_id TEXT NOT NULL,
+	-- The device ID to send the message to.
+	device_id TEXT NOT NULL,
+	-- The event content JSON.
+	content TEXT NOT NULL,
+	-- The token that was supplied to the /sync at the time that this
+	-- message was included in a sync response, or NULL if we haven't
+	-- included it in a /sync response yet.
+	sent_by_token TEXT
+);
+`
+
+const insertSendToDeviceMessageSQL = `
+	INSERT INTO syncapi_send_to_device (user_id, device_id, content)
+	  VALUES ($1, $2, $3)
+`
+
+const countSendToDeviceMessagesSQL = `
+	SELECT COUNT(*)
+	  FROM syncapi_send_to_device
+	  WHERE user_id = $1 AND device_id = $2
+`
+
+const selectSendToDeviceMessagesSQL = `
+	SELECT id, user_id, device_id, content, sent_by_token
+	  FROM syncapi_send_to_device
+	  WHERE user_id = $1 AND device_id = $2
+	  ORDER BY id DESC
+`
+
+const updateSentSendToDeviceMessagesSQL = `
+	UPDATE syncapi_send_to_device SET sent_by_token = $1
+	  WHERE id = ANY($2)
+`
+
+const deleteSendToDeviceMessagesSQL = `
+	DELETE FROM syncapi_send_to_device WHERE id = ANY($1)
+`
+
+type sendToDeviceStatements struct {
+	insertSendToDeviceMessageStmt      *sql.Stmt
+	countSendToDeviceMessagesStmt      *sql.Stmt
+	selectSendToDeviceMessagesStmt     *sql.Stmt
+	updateSentSendToDeviceMessagesStmt *sql.Stmt
+	deleteSendToDeviceMessagesStmt     *sql.Stmt
+}
+
+func NewPostgresSendToDeviceTable(db *sql.DB) (tables.SendToDevice, error) {
+	s := &sendToDeviceStatements{}
+	_, err := db.Exec(sendToDeviceSchema)
+	if err != nil {
+		return nil, err
+	}
+	if s.insertSendToDeviceMessageStmt, err = db.Prepare(insertSendToDeviceMessageSQL); err != nil {
+		return nil, err
+	}
+	if s.countSendToDeviceMessagesStmt, err = db.Prepare(countSendToDeviceMessagesSQL); err != nil {
+		return nil, err
+	}
+	if s.selectSendToDeviceMessagesStmt, err = db.Prepare(selectSendToDeviceMessagesSQL); err != nil {
+		return nil, err
+	}
+	if s.updateSentSendToDeviceMessagesStmt, err = db.Prepare(updateSentSendToDeviceMessagesSQL); err != nil {
+		return nil, err
+	}
+	if s.deleteSendToDeviceMessagesStmt, err = db.Prepare(deleteSendToDeviceMessagesSQL); err != nil {
+		return nil, err
+	}
+	return s, nil
+}
+
+func (s *sendToDeviceStatements) InsertSendToDeviceMessage(
+	ctx context.Context, txn *sql.Tx, userID, deviceID, content string,
+) (err error) {
+	_, err = internal.TxStmt(txn, s.insertSendToDeviceMessageStmt).ExecContext(ctx, userID, deviceID, content)
+	return
+}
+
+func (s *sendToDeviceStatements) CountSendToDeviceMessages(
+	ctx context.Context, txn *sql.Tx, userID, deviceID string,
+) (count int, err error) {
+	row := internal.TxStmt(txn, s.countSendToDeviceMessagesStmt).QueryRowContext(ctx, userID, deviceID)
+	if err = row.Scan(&count); err != nil {
+		return
+	}
+	return count, nil
+}
+
+func (s *sendToDeviceStatements) SelectSendToDeviceMessages(
+	ctx context.Context, txn *sql.Tx, userID, deviceID string,
+) (events []types.SendToDeviceEvent, err error) {
+	rows, err := internal.TxStmt(txn, s.selectSendToDeviceMessagesStmt).QueryContext(ctx, userID, deviceID)
+	if err != nil {
+		return
+	}
+	defer internal.CloseAndLogIfError(ctx, rows, "SelectSendToDeviceMessages: rows.close() failed")
+
+	for rows.Next() {
+		var id types.SendToDeviceNID
+		var userID, deviceID, content string
+		var sentByToken *string
+		if err = rows.Scan(&id, &userID, &deviceID, &content, &sentByToken); err != nil {
+			return
+		}
+		event := types.SendToDeviceEvent{
+			ID:       id,
+			UserID:   userID,
+			DeviceID: deviceID,
+		}
+		if err = json.Unmarshal([]byte(content), &event.SendToDeviceEvent); err != nil {
+			return
+		}
+		if sentByToken != nil {
+			if token, err := types.NewStreamTokenFromString(*sentByToken); err == nil {
+				event.SentByToken = &token
+			}
+		}
+		events = append(events, event)
+	}
+
+	return events, rows.Err()
+}
+
+func (s *sendToDeviceStatements) UpdateSentSendToDeviceMessages(
+	ctx context.Context, txn *sql.Tx, token string, nids []types.SendToDeviceNID,
+) (err error) {
+	_, err = txn.Stmt(s.updateSentSendToDeviceMessagesStmt).ExecContext(ctx, token, pq.Array(nids))
+	return
+}
+
+func (s *sendToDeviceStatements) DeleteSendToDeviceMessages(
+	ctx context.Context, txn *sql.Tx, nids []types.SendToDeviceNID,
+) (err error) {
+	_, err = txn.Stmt(s.deleteSendToDeviceMessagesStmt).ExecContext(ctx, pq.Array(nids))
+	return
+}

syncapi/storage/postgres/syncserver.go

@@ -69,6 +69,10 @@ func NewDatabase(dbDataSourceName string, dbProperties internal.DbProperties) (*
 	if err != nil {
 		return nil, err
 	}
+	sendToDevice, err := NewPostgresSendToDeviceTable(d.db)
+	if err != nil {
+		return nil, err
+	}
 	d.Database = shared.Database{
 		DB:                  d.db,
 		Invites:             invites,
@@ -77,6 +81,8 @@ func NewDatabase(dbDataSourceName string, dbProperties internal.DbProperties) (*
 		Topology:            topology,
 		CurrentRoomState:    currState,
 		BackwardExtremities: backwardExtremities,
+		SendToDevice:        sendToDevice,
+		SendToDeviceWriter:  internal.NewTransactionWriter(),
 		EDUCache:            cache.New(),
 	}
 	return &d, nil

syncapi/storage/shared/syncserver.go

@@ -1,3 +1,17 @@
+// Copyright 2020 The Matrix.org Foundation C.I.C.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
 package shared
 
 import (
@@ -27,6 +41,8 @@ type Database struct {
 	Topology            tables.Topology
 	CurrentRoomState    tables.CurrentRoomState
 	BackwardExtremities tables.BackwardsExtremities
+	SendToDevice        tables.SendToDevice
+	SendToDeviceWriter  *internal.TransactionWriter
 	EDUCache            *cache.EDUCache
 }
 
@@ -89,6 +105,10 @@ func (d *Database) RemoveTypingUser(
 	return types.StreamPosition(d.EDUCache.RemoveUser(userID, roomID))
 }
 
+func (d *Database) AddSendToDevice() types.StreamPosition {
+	return types.StreamPosition(d.EDUCache.AddSendToDeviceMessage())
+}
+
 func (d *Database) SetTypingTimeoutCallback(fn cache.TimeoutCallbackFn) {
 	d.EDUCache.SetTimeoutCallback(fn)
 }
@@ -528,14 +548,14 @@ func (d *Database) addEDUDeltaToResponse(
 }
 
 func (d *Database) IncrementalSync(
-	ctx context.Context,
+	ctx context.Context, res *types.Response,
 	device authtypes.Device,
 	fromPos, toPos types.StreamingToken,
 	numRecentEventsPerRoom int,
 	wantFullState bool,
 ) (*types.Response, error) {
 	nextBatchPos := fromPos.WithUpdates(toPos)
-	res := types.NewResponse(nextBatchPos)
+	res.NextBatch = nextBatchPos.String()
 
 	var joinedRoomIDs []string
 	var err error
@@ -568,12 +588,12 @@ func (d *Database) IncrementalSync(
 
 // getResponseWithPDUsForCompleteSync creates a response and adds all PDUs needed
 // to it. It returns toPos and joinedRoomIDs for use of adding EDUs.
+// nolint:nakedret
 func (d *Database) getResponseWithPDUsForCompleteSync(
-	ctx context.Context,
+	ctx context.Context, res *types.Response,
 	userID string,
 	numRecentEventsPerRoom int,
 ) (
-	res *types.Response,
 	toPos types.StreamingToken,
 	joinedRoomIDs []string,
 	err error,
@@ -604,7 +624,7 @@ func (d *Database) getResponseWithPDUsForCompleteSync(
 		To:   toPos.PDUPosition(),
 	}
 
-	res = types.NewResponse(toPos)
+	res.NextBatch = toPos.String()
 
 	// Extract room state and recent events for all rooms the user is joined to.
 	joinedRoomIDs, err = d.CurrentRoomState.SelectRoomIDsWithMembership(ctx, txn, userID, gomatrixserverlib.Join)
@@ -662,14 +682,15 @@ func (d *Database) getResponseWithPDUsForCompleteSync(
 	}
 
 	succeeded = true
-	return res, toPos, joinedRoomIDs, err
+	return //res, toPos, joinedRoomIDs, err
 }
 
 func (d *Database) CompleteSync(
-	ctx context.Context, device authtypes.Device, numRecentEventsPerRoom int,
+	ctx context.Context, res *types.Response,
+	device authtypes.Device, numRecentEventsPerRoom int,
 ) (*types.Response, error) {
-	res, toPos, joinedRoomIDs, err := d.getResponseWithPDUsForCompleteSync(
-		ctx, device.UserID, numRecentEventsPerRoom,
+	toPos, joinedRoomIDs, err := d.getResponseWithPDUsForCompleteSync(
+		ctx, res, device.UserID, numRecentEventsPerRoom,
 	)
 	if err != nil {
 		return nil, err
@@ -1028,6 +1049,115 @@ func (d *Database) currentStateStreamEventsForRoom(
 	return s, nil
 }
 
+func (d *Database) SendToDeviceUpdatesWaiting(
+	ctx context.Context, userID, deviceID string,
+) (bool, error) {
+	count, err := d.SendToDevice.CountSendToDeviceMessages(ctx, nil, userID, deviceID)
+	if err != nil {
+		return false, err
+	}
+	return count > 0, nil
+}
+
+func (d *Database) AddSendToDeviceEvent(
+	ctx context.Context, txn *sql.Tx,
+	userID, deviceID, content string,
+) error {
+	return d.SendToDevice.InsertSendToDeviceMessage(
+		ctx, txn, userID, deviceID, content,
+	)
+}
+
+func (d *Database) StoreNewSendForDeviceMessage(
+	ctx context.Context, streamPos types.StreamPosition, userID, deviceID string, event gomatrixserverlib.SendToDeviceEvent,
+) (types.StreamPosition, error) {
+	j, err := json.Marshal(event)
+	if err != nil {
+		return streamPos, err
+	}
+	// Delegate the database write task to the SendToDeviceWriter. It'll guarantee
+	// that we don't lock the table for writes in more than one place.
+	err = d.SendToDeviceWriter.Do(d.DB, func(txn *sql.Tx) error {
+		return d.AddSendToDeviceEvent(
+			ctx, txn, userID, deviceID, string(j),
+		)
+	})
+	if err != nil {
+		return streamPos, err
+	}
+	return streamPos, nil
+}
+
+func (d *Database) SendToDeviceUpdatesForSync(
+	ctx context.Context,
+	userID, deviceID string,
+	token types.StreamingToken,
+) ([]types.SendToDeviceEvent, []types.SendToDeviceNID, []types.SendToDeviceNID, error) {
+	// First of all, get our send-to-device updates for this user.
+	events, err := d.SendToDevice.SelectSendToDeviceMessages(ctx, nil, userID, deviceID)
+	if err != nil {
+		return nil, nil, nil, fmt.Errorf("d.SendToDevice.SelectSendToDeviceMessages: %w", err)
+	}
+
+	// If there's nothing to do then stop here.
+	if len(events) == 0 {
+		return nil, nil, nil, nil
+	}
+
+	// Work out whether we need to update any of the database entries.
+	toReturn := []types.SendToDeviceEvent{}
+	toUpdate := []types.SendToDeviceNID{}
+	toDelete := []types.SendToDeviceNID{}
+	for _, event := range events {
+		if event.SentByToken == nil {
+			// If the event has no sent-by token yet then we haven't attempted to send
+			// it. Record the current requested sync token in the database.
+			toUpdate = append(toUpdate, event.ID)
+			toReturn = append(toReturn, event)
+			event.SentByToken = &token
+		} else if token.IsAfter(*event.SentByToken) {
+			// The event had a sync token, therefore we've sent it before. The current
+			// sync token is now after the stored one so we can assume that the client
+			// successfully completed the previous sync (it would re-request it otherwise)
+			// so we can remove the entry from the database.
+			toDelete = append(toDelete, event.ID)
+		} else {
+			// It looks like the sync is being re-requested, maybe it timed out or
+			// failed. Re-send any that should have been acknowledged by now.
+			toReturn = append(toReturn, event)
+		}
+	}
+
+	return toReturn, toUpdate, toDelete, nil
+}
+
+func (d *Database) CleanSendToDeviceUpdates(
+	ctx context.Context,
+	toUpdate, toDelete []types.SendToDeviceNID,
+	token types.StreamingToken,
+) (err error) {
+	if len(toUpdate) == 0 && len(toDelete) == 0 {
+		return nil
+	}
+	// If we need to write to the database then we'll ask the SendToDeviceWriter to
+	// do that for us. It'll guarantee that we don't lock the table for writes in
+	// more than one place.
+	err = d.SendToDeviceWriter.Do(d.DB, func(txn *sql.Tx) error {
+		// Delete any send-to-device messages marked for deletion.
+		if e := d.SendToDevice.DeleteSendToDeviceMessages(ctx, txn, toDelete); e != nil {
+			return fmt.Errorf("d.SendToDevice.DeleteSendToDeviceMessages: %w", e)
+		}
+
+		// Now update any outstanding send-to-device messages with the new sync token.
+		if e := d.SendToDevice.UpdateSentSendToDeviceMessages(ctx, txn, token.String(), toUpdate); e != nil {
+			return fmt.Errorf("d.SendToDevice.UpdateSentSendToDeviceMessages: %w", err)
+		}
+
+		return nil
+	})
+	return
+}
+
 // There may be some overlap where events in stateEvents are already in recentEvents, so filter
 // them out so we don't include them twice in the /sync response. They should be in recentEvents
 // only, so clients get to the correct state once they have rolled forward.

syncapi/storage/sqlite3/send_to_device_table.go

@@ -0,0 +1,172 @@
+// Copyright 2019-2020 The Matrix.org Foundation C.I.C.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package sqlite3
+
+import (
+	"context"
+	"database/sql"
+	"encoding/json"
+	"strings"
+
+	"github.com/matrix-org/dendrite/internal"
+	"github.com/matrix-org/dendrite/syncapi/storage/tables"
+	"github.com/matrix-org/dendrite/syncapi/types"
+)
+
+const sendToDeviceSchema = `
+-- Stores send-to-device messages.
+CREATE TABLE IF NOT EXISTS syncapi_send_to_device (
+	-- The ID that uniquely identifies this message.
+	id INTEGER PRIMARY KEY AUTOINCREMENT,
+	-- The user ID to send the message to.
+	user_id TEXT NOT NULL,
+	-- The device ID to send the message to.
+	device_id TEXT NOT NULL,
+	-- The event content JSON.
+	content TEXT NOT NULL,
+	-- The token that was supplied to the /sync at the time that this
+	-- message was included in a sync response, or NULL if we haven't
+	-- included it in a /sync response yet.
+	sent_by_token TEXT
+);
+`
+
+const insertSendToDeviceMessageSQL = `
+	INSERT INTO syncapi_send_to_device (user_id, device_id, content)
+	  VALUES ($1, $2, $3)
+`
+
+const countSendToDeviceMessagesSQL = `
+	SELECT COUNT(*)
+	  FROM syncapi_send_to_device
+	  WHERE user_id = $1 AND device_id = $2
+`
+
+const selectSendToDeviceMessagesSQL = `
+	SELECT id, user_id, device_id, content, sent_by_token
+	  FROM syncapi_send_to_device
+	  WHERE user_id = $1 AND device_id = $2
+	  ORDER BY id DESC
+`
+
+const updateSentSendToDeviceMessagesSQL = `
+	UPDATE syncapi_send_to_device SET sent_by_token = $1
+	  WHERE id IN ($2)
+`
+
+const deleteSendToDeviceMessagesSQL = `
+	DELETE FROM syncapi_send_to_device WHERE id IN ($1)
+`
+
+type sendToDeviceStatements struct {
+	insertSendToDeviceMessageStmt  *sql.Stmt
+	selectSendToDeviceMessagesStmt *sql.Stmt
+	countSendToDeviceMessagesStmt  *sql.Stmt
+}
+
+func NewSqliteSendToDeviceTable(db *sql.DB) (tables.SendToDevice, error) {
+	s := &sendToDeviceStatements{}
+	_, err := db.Exec(sendToDeviceSchema)
+	if err != nil {
+		return nil, err
+	}
+	if s.countSendToDeviceMessagesStmt, err = db.Prepare(countSendToDeviceMessagesSQL); err != nil {
+		return nil, err
+	}
+	if s.insertSendToDeviceMessageStmt, err = db.Prepare(insertSendToDeviceMessageSQL); err != nil {
+		return nil, err
+	}
+	if s.selectSendToDeviceMessagesStmt, err = db.Prepare(selectSendToDeviceMessagesSQL); err != nil {
+		return nil, err
+	}
+	return s, nil
+}
+
+func (s *sendToDeviceStatements) InsertSendToDeviceMessage(
+	ctx context.Context, txn *sql.Tx, userID, deviceID, content string,
+) (err error) {
+	_, err = internal.TxStmt(txn, s.insertSendToDeviceMessageStmt).ExecContext(ctx, userID, deviceID, content)
+	return
+}
+
+func (s *sendToDeviceStatements) CountSendToDeviceMessages(
+	ctx context.Context, txn *sql.Tx, userID, deviceID string,
+) (count int, err error) {
+	row := internal.TxStmt(txn, s.countSendToDeviceMessagesStmt).QueryRowContext(ctx, userID, deviceID)
+	if err = row.Scan(&count); err != nil {
+		return
+	}
+	return count, nil
+}
+
+func (s *sendToDeviceStatements) SelectSendToDeviceMessages(
+	ctx context.Context, txn *sql.Tx, userID, deviceID string,
+) (events []types.SendToDeviceEvent, err error) {
+	rows, err := internal.TxStmt(txn, s.selectSendToDeviceMessagesStmt).QueryContext(ctx, userID, deviceID)
+	if err != nil {
+		return
+	}
+	defer internal.CloseAndLogIfError(ctx, rows, "SelectSendToDeviceMessages: rows.close() failed")
+
+	for rows.Next() {
+		var id types.SendToDeviceNID
+		var userID, deviceID, content string
+		var sentByToken *string
+		if err = rows.Scan(&id, &userID, &deviceID, &content, &sentByToken); err != nil {
+			return
+		}
+		event := types.SendToDeviceEvent{
+			ID:       id,
+			UserID:   userID,
+			DeviceID: deviceID,
+		}
+		if err = json.Unmarshal([]byte(content), &event.SendToDeviceEvent); err != nil {
+			return
+		}
+		if sentByToken != nil {
+			if token, err := types.NewStreamTokenFromString(*sentByToken); err == nil {
+				event.SentByToken = &token
+			}
+		}
+		events = append(events, event)
+	}
+
+	return events, rows.Err()
+}
+
+func (s *sendToDeviceStatements) UpdateSentSendToDeviceMessages(
+	ctx context.Context, txn *sql.Tx, token string, nids []types.SendToDeviceNID,
+) (err error) {
+	query := strings.Replace(updateSentSendToDeviceMessagesSQL, "($2)", internal.QueryVariadic(1+len(nids)), 1)
+	params := make([]interface{}, 1+len(nids))
+	params[0] = token
+	for k, v := range nids {
+		params[k+1] = v
+	}
+	_, err = txn.ExecContext(ctx, query, params...)
+	return
+}
+
+func (s *sendToDeviceStatements) DeleteSendToDeviceMessages(
+	ctx context.Context, txn *sql.Tx, nids []types.SendToDeviceNID,
+) (err error) {
+	query := strings.Replace(deleteSendToDeviceMessagesSQL, "($1)", internal.QueryVariadic(len(nids)), 1)
+	params := make([]interface{}, 1+len(nids))
+	for k, v := range nids {
+		params[k] = v
+	}
+	_, err = txn.ExecContext(ctx, query, params...)
+	return
+}

syncapi/storage/sqlite3/syncserver.go

@@ -95,6 +95,10 @@ func (d *SyncServerDatasource) prepare() (err error) {
 	if err != nil {
 		return err
 	}
+	sendToDevice, err := NewSqliteSendToDeviceTable(d.db)
+	if err != nil {
+		return err
+	}
 	d.Database = shared.Database{
 		DB:                  d.db,
 		Invites:             invites,
@@ -103,6 +107,8 @@ func (d *SyncServerDatasource) prepare() (err error) {
 		BackwardExtremities: bwExtrem,
 		CurrentRoomState:    roomState,
 		Topology:            topology,
+		SendToDevice:        sendToDevice,
+		SendToDeviceWriter:  internal.NewTransactionWriter(),
 		EDUCache:            cache.New(),
 	}
 	return nil

syncapi/storage/storage_test.go

@@ -3,6 +3,7 @@ package storage_test
 import (
 	"context"
 	"crypto/ed25519"
+	"encoding/json"
 	"fmt"
 	"testing"
 	"time"
@@ -157,7 +158,8 @@ func TestSyncResponse(t *testing.T) {
 				from := types.NewStreamToken( // pretend we are at the penultimate event
 					positions[len(positions)-2], types.StreamPosition(0),
 				)
-				return db.IncrementalSync(ctx, testUserDeviceA, from, latest, 5, false)
+				res := types.NewResponse()
+				return db.IncrementalSync(ctx, res, testUserDeviceA, from, latest, 5, false)
 			},
 			WantTimeline: events[len(events)-1:],
 		},
@@ -169,8 +171,9 @@ func TestSyncResponse(t *testing.T) {
 				from := types.NewStreamToken( // pretend we are 10 events behind
 					positions[len(positions)-11], types.StreamPosition(0),
 				)
+				res := types.NewResponse()
 				// limit is set to 5
-				return db.IncrementalSync(ctx, testUserDeviceA, from, latest, 5, false)
+				return db.IncrementalSync(ctx, res, testUserDeviceA, from, latest, 5, false)
 			},
 			// want the last 5 events, NOT the last 10.
 			WantTimeline: events[len(events)-5:],
@@ -180,8 +183,9 @@ func TestSyncResponse(t *testing.T) {
 		{
 			Name: "CompleteSync limited",
 			DoSync: func() (*types.Response, error) {
+				res := types.NewResponse()
 				// limit set to 5
-				return db.CompleteSync(ctx, testUserDeviceA, 5)
+				return db.CompleteSync(ctx, res, testUserDeviceA, 5)
 			},
 			// want the last 5 events
 			WantTimeline: events[len(events)-5:],
@@ -193,7 +197,8 @@ func TestSyncResponse(t *testing.T) {
 		{
 			Name: "CompleteSync",
 			DoSync: func() (*types.Response, error) {
-				return db.CompleteSync(ctx, testUserDeviceA, len(events)+1)
+				res := types.NewResponse()
+				return db.CompleteSync(ctx, res, testUserDeviceA, len(events)+1)
 			},
 			WantTimeline: events,
 			// We want no state at all as that field in /sync is the delta between the token (beginning of time)
@@ -234,7 +239,8 @@ func TestGetEventsInRangeWithPrevBatch(t *testing.T) {
 		positions[len(positions)-2], types.StreamPosition(0),
 	)
 
-	res, err := db.IncrementalSync(ctx, testUserDeviceA, from, latest, 5, false)
+	res := types.NewResponse()
+	res, err = db.IncrementalSync(ctx, res, testUserDeviceA, from, latest, 5, false)
 	if err != nil {
 		t.Fatalf("failed to IncrementalSync with latest token")
 	}
@@ -512,6 +518,89 @@ func TestGetEventsInRangeWithEventsInsertedLikeBackfill(t *testing.T) {
 	}
 }
 
+func TestSendToDeviceBehaviour(t *testing.T) {
+	//t.Parallel()
+	db := MustCreateDatabase(t)
+
+	// At this point there should be no messages. We haven't sent anything
+	// yet.
+	events, updates, deletions, err := db.SendToDeviceUpdatesForSync(ctx, "alice", "one", types.NewStreamToken(0, 0))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(events) != 0 || len(updates) != 0 || len(deletions) != 0 {
+		t.Fatal("first call should have no updates")
+	}
+	err = db.CleanSendToDeviceUpdates(context.Background(), updates, deletions, types.NewStreamToken(0, 0))
+	if err != nil {
+		return
+	}
+
+	// Try sending a message.
+	streamPos, err := db.StoreNewSendForDeviceMessage(ctx, types.StreamPosition(0), "alice", "one", gomatrixserverlib.SendToDeviceEvent{
+		Sender:  "bob",
+		Type:    "m.type",
+		Content: json.RawMessage("{}"),
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// At this point we should get exactly one message. We're sending the sync position
+	// that we were given from the update and the send-to-device update will be updated
+	// in the database to reflect that this was the sync position we sent the message at.
+	events, updates, deletions, err = db.SendToDeviceUpdatesForSync(ctx, "alice", "one", types.NewStreamToken(0, streamPos))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(events) != 1 || len(updates) != 1 || len(deletions) != 0 {
+		t.Fatal("second call should have one update")
+	}
+	err = db.CleanSendToDeviceUpdates(context.Background(), updates, deletions, types.NewStreamToken(0, streamPos))
+	if err != nil {
+		return
+	}
+
+	// At this point we should still have one message because we haven't progressed the
+	// sync position yet. This is equivalent to the client failing to /sync and retrying
+	// with the same position.
+	events, updates, deletions, err = db.SendToDeviceUpdatesForSync(ctx, "alice", "one", types.NewStreamToken(0, streamPos))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(events) != 1 || len(updates) != 0 || len(deletions) != 0 {
+		t.Fatal("third call should have one update still")
+	}
+	err = db.CleanSendToDeviceUpdates(context.Background(), updates, deletions, types.NewStreamToken(0, streamPos))
+	if err != nil {
+		return
+	}
+
+	// At this point we should now have no updates, because we've progressed the sync
+	// position. Therefore the update from before will not be sent again.
+	events, updates, deletions, err = db.SendToDeviceUpdatesForSync(ctx, "alice", "one", types.NewStreamToken(0, streamPos+1))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(events) != 0 || len(updates) != 0 || len(deletions) != 1 {
+		t.Fatal("fourth call should have no updates")
+	}
+	err = db.CleanSendToDeviceUpdates(context.Background(), updates, deletions, types.NewStreamToken(0, streamPos+1))
+	if err != nil {
+		return
+	}
+
+	// At this point we should still have no updates, because no new updates have been
+	// sent.
+	events, updates, deletions, err = db.SendToDeviceUpdatesForSync(ctx, "alice", "one", types.NewStreamToken(0, streamPos+2))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(events) != 0 || len(updates) != 0 || len(deletions) != 0 {
+		t.Fatal("fifth call should have no updates")
+	}
+}
+
 func assertEventsEqual(t *testing.T, msg string, checkRoomID bool, gots []gomatrixserverlib.ClientEvent, wants []gomatrixserverlib.HeaderedEvent) {
 	if len(gots) != len(wants) {
 		t.Fatalf("%s response returned %d events, want %d", msg, len(gots), len(wants))

syncapi/storage/tables/interface.go

@@ -1,3 +1,17 @@
+// Copyright 2020 The Matrix.org Foundation C.I.C.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
 package tables
 
 import (
@@ -94,3 +108,28 @@ type BackwardsExtremities interface {
 	// DeleteBackwardExtremity removes a backwards extremity for a room, if one existed.
 	DeleteBackwardExtremity(ctx context.Context, txn *sql.Tx, roomID, knownEventID string) (err error)
 }
+
+// SendToDevice tracks send-to-device messages which are sent to individual
+// clients. Each message gets inserted into this table at the point that we
+// receive it from the EDU server.
+//
+// We're supposed to try and do our best to deliver send-to-device messages
+// once, but the only way that we can really guarantee that they have been
+// delivered is if the client successfully requests the next sync as given
+// in the next_batch. Each time the device syncs, we will request all of the
+// updates that either haven't been sent yet, along with all updates that we
+// *have* sent but we haven't confirmed to have been received yet. If it's the
+// first time we're sending a given update then we update the table to say
+// what the "since" parameter was when we tried to send it.
+//
+// When the client syncs again, if their "since" parameter is *later* than
+// the recorded one, we drop the entry from the DB as it's "sent". If the
+// sync parameter isn't later then we will keep including the updates in the
+// sync response, as the client is seemingly trying to repeat the same /sync.
+type SendToDevice interface {
+	InsertSendToDeviceMessage(ctx context.Context, txn *sql.Tx, userID, deviceID, content string) (err error)
+	SelectSendToDeviceMessages(ctx context.Context, txn *sql.Tx, userID, deviceID string) (events []types.SendToDeviceEvent, err error)
+	UpdateSentSendToDeviceMessages(ctx context.Context, txn *sql.Tx, token string, nids []types.SendToDeviceNID) (err error)
+	DeleteSendToDeviceMessages(ctx context.Context, txn *sql.Tx, nids []types.SendToDeviceNID) (err error)
+	CountSendToDeviceMessages(ctx context.Context, txn *sql.Tx, userID, deviceID string) (count int, err error)
+}

syncapi/sync/notifier.go

@@ -120,6 +120,18 @@ func (n *Notifier) OnNewEvent(
 	}
 }
 
+func (n *Notifier) OnNewSendToDevice(
+	userID string, deviceIDs []string,
+	posUpdate types.StreamingToken,
+) {
+	n.streamLock.Lock()
+	defer n.streamLock.Unlock()
+	latestPos := n.currPos.WithUpdates(posUpdate)
+	n.currPos = latestPos
+
+	n.wakeupUserDevice(userID, deviceIDs, latestPos)
+}
+
 // GetListener returns a UserStreamListener that can be used to wait for
 // updates for a user. Must be closed.
 // notify for anything before sincePos
@@ -189,8 +201,8 @@ func (n *Notifier) wakeupUsers(userIDs []string, newPos types.StreamingToken) {
 // wakeupUserDevice will wake up the sync stream for a specific user device. Other
 // device streams will be left alone.
 // nolint:unused
-func (n *Notifier) wakeupUserDevice(userDevices map[string]string, newPos types.StreamingToken) {
-	for userID, deviceID := range userDevices {
+func (n *Notifier) wakeupUserDevice(userID string, deviceIDs []string, newPos types.StreamingToken) {
+	for _, deviceID := range deviceIDs {
 		if stream := n.fetchUserDeviceStream(userID, deviceID, false); stream != nil {
 			stream.Broadcast(newPos) // wake up all goroutines Wait()ing on this stream
 		}

syncapi/sync/notifier_test.go

@@ -172,7 +172,7 @@ func TestCorrectStreamWakeup(t *testing.T) {
 	time.Sleep(1 * time.Second)
 
 	wake := "two"
-	n.wakeupUserDevice(map[string]string{alice: wake}, syncPositionAfter)
+	n.wakeupUserDevice(alice, []string{wake}, syncPositionAfter)
 
 	if result := <-awoken; result != wake {
 		t.Fatalf("expected to wake %q, got %q", wake, result)

syncapi/sync/requestpool.go

@@ -1,4 +1,6 @@
 // Copyright 2017 Vector Creations Ltd
+// Copyright 2017-2018 New Vector Ltd
+// Copyright 2019-2020 The Matrix.org Foundation C.I.C.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -15,6 +17,7 @@
 package sync
 
 import (
+	"context"
 	"net/http"
 	"time"
 
@@ -54,17 +57,18 @@ func (rp *RequestPool) OnIncomingSyncRequest(req *http.Request, device *authtype
 			JSON: jsonerror.Unknown(err.Error()),
 		}
 	}
+
 	logger := util.GetLogger(req.Context()).WithFields(log.Fields{
-		"userID":   device.UserID,
-		"deviceID": device.ID,
-		"since":    syncReq.since,
-		"timeout":  syncReq.timeout,
-		"limit":    syncReq.limit,
+		"user_id":   device.UserID,
+		"device_id": device.ID,
+		"since":     syncReq.since,
+		"timeout":   syncReq.timeout,
+		"limit":     syncReq.limit,
 	})
 
 	currPos := rp.notifier.CurrentPosition()
 
-	if shouldReturnImmediately(syncReq) {
+	if rp.shouldReturnImmediately(syncReq) {
 		syncData, err = rp.currentSyncForUser(*syncReq, currPos)
 		if err != nil {
 			logger.WithError(err).Error("rp.currentSyncForUser failed")
@@ -116,7 +120,6 @@ func (rp *RequestPool) OnIncomingSyncRequest(req *http.Request, device *authtype
 		// response. This ensures that we don't waste the hard work
 		// of calculating the sync only to get timed out before we
 		// can respond
-
 		syncData, err = rp.currentSyncForUser(*syncReq, currPos)
 		if err != nil {
 			logger.WithError(err).Error("rp.currentSyncForUser failed")
@@ -134,19 +137,59 @@ func (rp *RequestPool) OnIncomingSyncRequest(req *http.Request, device *authtype
 }
 
 func (rp *RequestPool) currentSyncForUser(req syncRequest, latestPos types.StreamingToken) (res *types.Response, err error) {
-	// TODO: handle ignored users
-	if req.since == nil {
-		res, err = rp.db.CompleteSync(req.ctx, req.device, req.limit)
-	} else {
-		res, err = rp.db.IncrementalSync(req.ctx, req.device, *req.since, latestPos, req.limit, req.wantFullState)
+	res = types.NewResponse()
+
+	since := types.NewStreamToken(0, 0)
+	if req.since != nil {
+		since = *req.since
 	}
 
+	// See if we have any new tasks to do for the send-to-device messaging.
+	events, updates, deletions, err := rp.db.SendToDeviceUpdatesForSync(req.ctx, req.device.UserID, req.device.ID, since)
+	if err != nil {
+		return nil, err
+	}
+
+	// TODO: handle ignored users
+	if req.since == nil {
+		res, err = rp.db.CompleteSync(req.ctx, res, req.device, req.limit)
+	} else {
+		res, err = rp.db.IncrementalSync(req.ctx, res, req.device, *req.since, latestPos, req.limit, req.wantFullState)
+	}
 	if err != nil {
 		return
 	}
 
 	accountDataFilter := gomatrixserverlib.DefaultEventFilter() // TODO: use filter provided in req instead
 	res, err = rp.appendAccountData(res, req.device.UserID, req, latestPos.PDUPosition(), &accountDataFilter)
+	if err != nil {
+		return
+	}
+
+	// Before we return the sync response, make sure that we take action on
+	// any send-to-device database updates or deletions that we need to do.
+	// Then add the updates into the sync response.
+	if len(updates) > 0 || len(deletions) > 0 {
+		// Handle the updates and deletions in the database.
+		err = rp.db.CleanSendToDeviceUpdates(context.Background(), updates, deletions, since)
+		if err != nil {
+			return
+		}
+	}
+	if len(events) > 0 {
+		// Add the updates into the sync response.
+		for _, event := range events {
+			res.ToDevice.Events = append(res.ToDevice.Events, event.SendToDeviceEvent)
+		}
+
+		// Get the next_batch from the sync response and increase the
+		// EDU counter.
+		if pos, perr := types.NewStreamTokenFromString(res.NextBatch); perr == nil {
+			pos.Positions[1]++
+			res.NextBatch = pos.String()
+		}
+	}
+
 	return
 }
 
@@ -238,6 +281,10 @@ func (rp *RequestPool) appendAccountData(
 // shouldReturnImmediately returns whether the /sync request is an initial sync,
 // or timeout=0, or full_state=true, in any of the cases the request should
 // return immediately.
-func shouldReturnImmediately(syncReq *syncRequest) bool {
-	return syncReq.since == nil || syncReq.timeout == 0 || syncReq.wantFullState
+func (rp *RequestPool) shouldReturnImmediately(syncReq *syncRequest) bool {
+	if syncReq.since == nil || syncReq.timeout == 0 || syncReq.wantFullState {
+		return true
+	}
+	waiting, werr := rp.db.SendToDeviceUpdatesWaiting(context.TODO(), syncReq.device.UserID, syncReq.device.ID)
+	return werr == nil && waiting
 }

syncapi/syncapi.go

@@ -78,7 +78,14 @@ func SetupSyncAPIComponent(
 		base.Cfg, base.KafkaConsumer, notifier, syncDB,
 	)
 	if err = typingConsumer.Start(); err != nil {
-		logrus.WithError(err).Panicf("failed to start typing server consumer")
+		logrus.WithError(err).Panicf("failed to start typing consumer")
+	}
+
+	sendToDeviceConsumer := consumers.NewOutputSendToDeviceEventConsumer(
+		base.Cfg, base.KafkaConsumer, notifier, syncDB,
+	)
+	if err = sendToDeviceConsumer.Start(); err != nil {
+		logrus.WithError(err).Panicf("failed to start send-to-device consumer")
 	}
 
 	routing.Setup(base.PublicAPIMux, requestPool, syncDB, deviceDB, federation, rsAPI, cfg)

syncapi/types/types.go

@@ -296,13 +296,14 @@ type Response struct {
 		Invite map[string]InviteResponse `json:"invite"`
 		Leave  map[string]LeaveResponse  `json:"leave"`
 	} `json:"rooms"`
+	ToDevice struct {
+		Events []gomatrixserverlib.SendToDeviceEvent `json:"events"`
+	} `json:"to_device"`
 }
 
 // NewResponse creates an empty response with initialised maps.
-func NewResponse(token StreamingToken) *Response {
-	res := Response{
-		NextBatch: token.String(),
-	}
+func NewResponse() *Response {
+	res := Response{}
 	// Pre-initialise the maps. Synapse will return {} even if there are no rooms under a specific section,
 	// so let's do the same thing. Bonus: this means we can't get dreaded 'assignment to entry in nil map' errors.
 	res.Rooms.Join = make(map[string]JoinResponse)
@@ -315,6 +316,7 @@ func NewResponse(token StreamingToken) *Response {
 	//       This also applies to NewJoinResponse, NewInviteResponse and NewLeaveResponse.
 	res.AccountData.Events = make([]gomatrixserverlib.ClientEvent, 0)
 	res.Presence.Events = make([]gomatrixserverlib.ClientEvent, 0)
+	res.ToDevice.Events = make([]gomatrixserverlib.SendToDeviceEvent, 0)
 
 	return &res
 }
@@ -326,7 +328,8 @@ func (r *Response) IsEmpty() bool {
 		len(r.Rooms.Invite) == 0 &&
 		len(r.Rooms.Leave) == 0 &&
 		len(r.AccountData.Events) == 0 &&
-		len(r.Presence.Events) == 0
+		len(r.Presence.Events) == 0 &&
+		len(r.ToDevice.Events) == 0
 }
 
 // JoinResponse represents a /sync response for a room which is under the 'join' key.
@@ -393,3 +396,13 @@ func NewLeaveResponse() *LeaveResponse {
 	res.Timeline.Events = make([]gomatrixserverlib.ClientEvent, 0)
 	return &res
 }
+
+type SendToDeviceNID int
+
+type SendToDeviceEvent struct {
+	gomatrixserverlib.SendToDeviceEvent
+	ID          SendToDeviceNID
+	UserID      string
+	DeviceID    string
+	SentByToken *StreamingToken
+}